Oil and gas wells are ordinarily completed by first cementing metallic casing stringers in the borehole. Depending on the properties of the formation (e.g., formation porosity), a dual casing string may be employed, for example, including a smaller diameter string deployed internal to a larger diameter string. In such dual-string wellbores, the internal string is commonly cemented to the larger diameter string (i.e., the annular region between the first and second strings is filled or partially filled with cement).
When oil and gas wells are no longer commercially viable, they must be abandoned in accordance with local government regulations. These regulations vary from one jurisdiction to another; however, they generally require one or more permanent barriers to isolate the wellbore. In certain jurisdictions, well abandonment requires a length (e.g., about 50 meters) of the wellbore casing string to be removed prior to filling the wellbore with a cement plug. The casing string is commonly removed via a milling operation that employs a downhole milling tool having a plurality of circumferentially spaced milling/cutting blades that extend radially outward from a tool body. During a typical milling operation, the milling tool is deployed on a tool string and rotated in the wellbore such that the blades make a circumferential cut in the metallic casing string. The tool string is then urged downhole while rotation continues so as to axially mill the casing string to the desired length.
While such milling tools are commonly employed in downhole milling operations, their use is not without certain drawbacks. For example, milling a dual-string wellbore typically requires the tool string to be tripped out of the wellbore after milling the smaller diameter string so as to install larger diameter blades. A separate drilling operation may also be required to remove the cement layer located between the inner and outer strings. These multiple operations and trips are both time consuming and expensive and therefore are undesirable.
The use of larger diameter milling blades can also be problematic in that the larger blades are subject to increased shear and torsional loads and therefore more prone to failure (e.g., via fracturing or circumferentially wrapping around the tool body). Moreover, for this reason, the use of larger diameter milling blades does not generally enable simultaneous removal of the cement layer and one or both of the casing strings. Larger diameter blades are also difficult to fully collapse into a tool body. Hence, tripping a tool having larger diameter blades can be problematic as the larger blades may hang up in smaller diameter casing (even when collapsed into the tool body).
As a result, there is a need for a milling tool capable of being deployed in a dual-string wellbore in a single trip, and preferably capable of simultaneously removing a cement layer and milling at least one casing string.